In order to create faster and more powerful integrated circuits, circuit designers are increasing the number and decreasing the size of circuit elements that are placed on an integrated circuit. With conventional photolithography, the minimum size of an object that can be created on a silicon wafer depends directly on the wavelength of light used to expose the wafer and inversely on the numerical aperture of the lens through which the light that exposes the wafer is passed. Because the costs associated with decreasing the illumination wavelength or increasing the numerical aperture can be prohibitive, chip manufacturers are continually looking for techniques that can create smaller objects on a wafer using existing photolithographic equipment.
One of the most powerful techniques for increasing the density of an integrated circuit with existing photolithographic equipment is with the use of phase shifters. As discovered by Marc Levenson of IBM and others, the phase of the light that strikes a wafer can be manipulated to destructively interfere at desired locations on the wafer in order to enhance image contrast and reduce diffraction effects that occur when the light passes through a pattern of opaque areas on a semiconductor mask. In addition, by selectively placing phase shifters on the mask, subwavelength features can be created on the wafer to form circuit elements.
While the use of phase shifting structures on a photolithographic mask allows increased contrast and the creation of subwavelength features using existing photolithographic equipment, the phase shifters are relatively expensive to create and may introduce errors into the mask. Therefore, there is a need for a method and apparatus for optimizing the creation of phase shifters on a mask that minimizes the possibility of errors and facilitates the production of the mask.